The Gulf Stream, the warm current that brings the east coast of Florida the mixed blessings of abundant swordfish, mild winters and stronger hurricanes, may be weakening because of climate change.

Visible from the air as a ribbon of cobalt blue water a few miles off the coast, the Gulf Stream forms part of a clockwise system of currents that transports warm water from the tropics up the east coast and across the Atlantic to northwestern Europe. In the frigid climate near Greenland, the water cools, sinks and flows south again, rolling through the deep ocean toward the tropics.

This marine circulatory system has reached its weakest point in 1,600 years, recent studies show, having lost about 15% of its strength since the mid-20th century. Scientists disagree on whether climate change or natural cycles account for the slowdown. But a…